FS Singlespeed help

I'm setting up my old Pace 405 frame to run singlespeed, and just can't get the chain to run smoothly.
I've got a Yess ETR/D chain tensioner ( http://labs.yesspro.com/products/etr-d) fitted, and a single Surly cog, with a variety of Surly spacers. Whatever combination of cog positioning I try, the chain clunks irregularly when I pedal (just along level road). I've tried setting the spaces from one extreme to the other (from the chain falling off inside the cog, to the chain falling off outside), and just about every combination in between, but whatever I do it "clunks".
anyone got any ideas?

yep new chain, sprocket and chainring. I wondered about a stiff link, but it seems to be very irregular. I'll check the whole chain again. maybe take a couple of links out? (sadly Yess don't provide any setup guide)

Is the joining link in te chain wider than the rest of them? Clunking as it goes through whatever bit its hitting off at the time - determined by where the chain is sitting on the cogs at that particular moment in the space time continuum. Either that or its a gammy flux capacitor ken.

Yes i have but i can't see what's happening....with the bike upsidedown it barely clunks (more clicks!). With me on it....I can't see what's going on!
Chain tension seems like a good place to start maybe?

The link for the tensioner doesn't appear to work for me but, if it's a FS bike is the chain tension not constatly changing as the suspension compresses, or does the frame have a pivot around the bottom bracket?

you can feel it through the pedals, definitely.
Hicksy, the tensioner is made specifically made for FS frames, and looks, and works, like a rear mech., so it takes up the slack as the chain grows.
I'll have a play today and take a couple of links out of the chain, and maybe see if I can put more tension in the tensioner (the spring position is adjustable)
thanks for the ideas so far!

Psling, I'm starting to conclude that the chain isn't compatible with the cog...I've spend some time today fiddling, and putting more tension in, but I'm still getting the "clunk". I'm starting to wonder if the chain is not sitting on the cog properly, and then slips onto it under pressure - which would explain why it doesn't clunk with no pressure.

so that should all be fine. I guess the only thing I don't know about is the pulley wheel on the tensioner...I think I read somewhere that it's a "Shimano standard"..whatever that is. maybe that's not OK with a 3/32" chain

The shimano jockey will likely be 3/32 too so all compatible.
What size cog is it? If smaller than 16T (say for DH use) you may not be getting good chain wrap causing a slip.
Or maybe the freehub needs servicing... Maybe where davidtaylforth's 'chain' of thought is going...?

it's a 16T cog. I'd not thought about the freehub being knackered...it has been sitting round unused for a while. I could also quite quickly swap wheels for one that I know has a sound hub.
thanks guys...I'll let you know how I get on

I found my trance worked perfect with a 18 tooth but slipped with a 20 and a 17 tooth. Round here I ran it with an 18 but at SSWC I ran it with 20 and a slightly too short chain and loads of air in the shock so it never got to full travel / longest chain point.